Research On Removal Behavior Of Typical PPCPs In Vertical Flow Constructed Wetlands

In recent years,a variety of drugs and personal care products?Pharmaceutical and Personal Care Products,referred to as PPCPs?have been detected in rivers of China and its concentrations range in the ng/L^?g/L level.In spite of low concentration and short half-life,the increasing inputs of PPCPs into the water make them "persistent" in the environment.PPCPs in the aquatic environment have superposition amplification effect and induce a highly resistant bacteria,such as super bacteria.PPCPs,being one of major pollutants in water environment,can enter human or animal body through the social circulation of water,leading to generation of ecological toxicity.Sulfamethoxazole?SMX?,a typical broad-spectrum antibiotics,is widely used for inhibition of sensitive bacteria which cause a variety of infectious diseases.However,the conventional sewage treatment to remove the effect is limited,so that untreated PPCPs into the natural water environment has been detected frequently.Artificial wetlands as an ecological sewage treatment technology,derived from natural wetlands,have been shown considerable removal on most of PPCPs.The removal of PPCPs by artificial wetlands is achieved by a series of combined effects such as matrix adsorption,microbial degradation,plant absorption,photolysis and so on.Compared with traditional sewage treatment technology,it has the characteristics of low cost and easy maintenance,and can be integrated with sponge city construction.Previous studies on the removal of PPCPs from constructed wetlands mainly focused on the overall removal effect or a factor in the system,while careful discussion of the various factors is limitted.In this paper,SMX as the target pollutant and three kinds of artificial wetland substrates?zeolite,ceramsite,slag?were selected to investigate the adsorption kinetics,adsorption temperature characteristics and influence factors.Based on stable operation of VFW?Vertical Flow Constructed Wetland?,the contribution of matrix adsorption and microbial degradation were discussed and the fate of SMX in artificial wetlands was further investigated under the operating conditions of different hydraulic retention time?HRT?and SMX loading rate.Finally,the ecological dynamcs model of SMX in vertical flow was established to simulate and verify the experimental results.The adsorption of antibiotic organic pollutants by three kinds of artificial wetland substrates showed the following order: slag>ceramsite>zeolite.Besides,the results showed that the first 4 h was the rapid adsorption stage,and the adsorption equilibrium was reached at 12 h.The adsorption kinetic analysis showed that the adsorption process was more consistent with the first order kinetic model and second order kinetic model instead of ion diffusion model,which indicated that the adsorption was controlled by physical and chemical process.In addition,the adsorption capacity of SMX and the remaining amount of SMX raised with the increase of SMX dosage during the isothermal adsorption process.Langmuir equation analysis showed that the adsorption process included physical adsorption and chemically adsorbed single molecule layer adsorption.The adsorption capacity of SMX in zeolite decreased with the increase of temperature?15³5??and raised later,then decreased again.The adsorption amounts of SMX by ceramsite and slag were likely to be consistent with variation trend of zeolite and the three different changes in speed were just led by the different physical and chemical properties of the matrixs.Acid or alkali are not conducive to the adsorption of SMX and the adsorption effect of the three substrates achieved the best in neutral conditions.Through the SEM characterization,it was confirmed from the microscopic point that the three matrices had an effective adsorption on SMX.In a well-run artificial wetland simulation column,different HRT of 8h,12 h,16h and 24 h were setted.The results showed that the removal of SMX was affected by HRT and 16 h was considered to be the optimal HRT for the best removal.With the change of SMX load?50,100,200,400,600 and 800 ?g/L?,the removal efficiency of sulfamethoxazole,which ranged between 81.73% and 99.74% with a average value of 92.80%,fistly increased and then decreased.In the process of flow from the liquid phase to the solid composite matrix?zeolite,ceramsite and slag?,slag showed higher adsorption on sulfamethoxazole by 31.76%,followed by 25.61% and 18.85% for zeolite and ceramsite.However,zeolite achieved greater microbiological degradation with the value of 3.40%,followed by 2.49% and 1.86% respectively for ceramsite and slag,on average.Based on analysis of the correlation coefficient between sulfamethoxazole removal rate and hydration index,the removal of sulfamethoxazole was significantly positively correlated with the degradation of COD and TN?P<0.01?,which indicated that sulfamethoxazole was potentially degraded by aerobic biodegradation.Based on the existing artificial wetland model and theoretical basis,the matrix adsorption,microbial degradation and wall adsorption rate were solved.Finally,the model of SMX removal model e?t?=I?t?-0.1327q-42.72C_e^0.350/tm was obtained.According to the comparison with the measured data,it was found that the measured test effluent was consistent with the trend of the simulated water flow.The error of the test removal rate and the model removal rate ranged from 0.17% to 5.34%,which indicated that the removal model of SMX,which is based on the conservation of material,well described the quantitative balance process and the relationship of the subsystem factors in the vertical flow artificial wetland.However,due to the diversity of hydraulic flow and the unmeasurable nature of internal retention in the constructed wetland system,the model is inevitably flawed and needs to be further verified and perfected.